Examples of the secondary battery include a nickel-metal hydride battery, a lead-acid battery, and a lithium-ion secondary battery.
Such secondary batteries are used as power sources in portable information terminals typified by mobile phones. In particular, lithium-ion secondary batteries have been actively developed because capacity thereof can be increased and size thereof can be reduced.
In a lithium-ion secondary battery, as a positive electrode active material, a phosphate compound having an olivine structure and containing lithium and iron, manganese, cobalt, or nickel, such as lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), and lithium nickel phosphate (LiNiPO4), which are disclosed in Patent Document 1, has been known.
In addition, as described in Non-Patent Document 1 and Non-Patent Document 2, a method for measuring the valence of a metal in a metal oxide, and the like by electron energy loss spectroscopy (EELS) is known.